This invention relates to a semiconductor light emitting device having increased current capacity and increased brightness. In particular, this invention relates to a light emitting diode (LED) having a configuration that includes a highly thermally conductive surface mount package that enables the use of higher drives currents and thus produces increased brightness.
Light emitting diodes (LEDs) that are currently used have limitations. For example, at relatively low drive currents, the amount of flux an LED produces is directly related to its drive current. For most applications, it is desirable to drive LEDs at as high a current as possible in order to maximize the brightness of their emission. But, at higher drive currents, an LED generates a greater amount of heat. And, as the temperature of the LED die increases, the LED becomes less efficient and thus, its brightness is reduced. As an LED die experiences increasing temperatures during operation, not only can its efficiency be reduced, but it can also create reliability problems within the LED structure. Further, increased die temperature may also lead to a wavelength shift in the LED. The temperature of the environment surrounding the LED will also contribute to the temperature of the LED die and thus, these temperature effects. Therefore, there is a need for this heat to be dissipated.
Generally, the primary means through which an LED dissipates heat is through its package. Of the packaging choices available, through-hole leaded packages and surface mount packages are the most common. Of these two choices, through-hole leaded packages can accommodate large lead frames and therefore generally provide better heat conductivity. But, use of through-hole leaded packages in circuit board assemblies requires the use of a costly and difficult to control wave solder process. Surface mount packaging is often more desirable from a circuit board assembly manufacturing standpoint, because it can be used with a more precise reflow solder process. But, prior to the present invention, surface mount packages have not combined the use of high thermal conductivity lead frame designs for use at high drive currents (that is, applications at which the LED is to be driven at a current in excess of 50 mA), with improved die attach methods for operation in high temperature environments (eliminating the wire bond connection).
The drive current of an LED is also limited by the mounting of the semiconductor die within the package. In a traditional configuration, the die is positioned on its substrate within the package on top of the cathode (or anode). A wire bond is used to establish contact between the die and the anode (or cathode). The die and wire bond are typically over-molded with an epoxy. The wire bond and epoxy, however, are also susceptible to deterioration due to the effects of heat. That is, as the epoxy heats and cools, the epoxy expands, causing stress in the wire. Eventually, the LED may fail due to stress and a break in the wire bond.
Alternatively, if a die is used which emits light from both the top and bottom surfaces (transmissive substrate), the die may be positioned on its side as a “flop-chip” (with substrate and light emitting component arranged side-by-side) or upside down as a “flip-chip” (with substrate on top of light emitting component). In both cases, the die is positioned between the cathode and anode, yet still produces a symmetrical pattern of light. In the “flop-chip” and “flip-chip” configuration, the wire bond is eliminated and solder bridges are used to establish contact between the die and the anode and cathode pads. While these configurations improve the reliability of the anode and cathode to die connections compared to use of a wire bond, they have not heretofore been used in high current (above 50 mA) LED devices.
These and other drawbacks exist with conventional LEDs.